DC voltage output is too high

by:Mainbon     2021-07-03
This failure is often caused by abnormalities in the voltage stabilization sampling and voltage stabilization control circuit. In the charger, the DC output, sampling resistor, error sampling amplifier, optocoupler, power control chip, etc. together form a closed control loop , Any problem will cause the voltage to rise.  Maintenance method: As the charger has an overvoltage protection circuit, if the output voltage is too high, the overvoltage protection circuit will be activated first. Therefore, when encountering this kind of failure, we can disconnect the overvoltage protection circuit to make this voltage protection circuit inoperative, and then measure the main voltage of the power supply at the moment of power-on. If the measured value is higher than the normal value by more than 1V, it indicates that the cause of the excessive output voltage is indeed in the control loop. At this time, you should focus on checking whether the sampling resistor is changed or damaged, and whether the precision reference voltage source (TL431) or the optocoupler (PC817) has poor performance, deterioration or damage. Among them, the precision reference voltage source (TL431) is extremely easy to damage. We can use the following method to distinguish the precision voltage regulator amplifier: connect the reference terminal (Ref) of TL431 to its cathode (Cathode), string a 1OkΩ resistor, and connect it to 5 ∨Voltage. If the voltage between the anode and the cathode is 2.5V, and it is still 2.5∨ after waiting for a while, it is a good tube, otherwise it is a bad tube. DC voltage output is too low. According to maintenance experience, in addition to the low output voltage caused by the voltage stabilization control circuit, there are also the following reasons:    (1) The output voltage terminal rectifier triode and the filter capacitor fail, which can be performed by substitution. judgment.   (2) The performance of the switching power tube is degraded, causing the switching tube to fail to conduct normally, which increases the internal resistance of the power supply and reduces the load capacity.   (3) The source of the switching power tube is usually connected to a resistor with a small resistance but a large power as an overcurrent protection detection resistor. The resistance of this resistor is generally 0.2~0.8Ω. Such as the resistance change value, open welding, poor contact will also cause the output voltage to be too low.   (4) The high-frequency pulse transformer is not good, not only causes the output voltage to drop, but also causes insufficient excitation of the switching power tube, which causes repeated damage to the switching tube.  (5) The high voltage DC filter capacitor is bad, resulting in poor load carrying capacity of the power supply.   (6) Poor contact of the power output line, with a certain contact resistance, causing the output voltage to be too low.  (7) The grid voltage is too low. Although the charger can still output the rated charging voltage at low levels, when the grid voltage is lower than the charger's minimum voltage limit, the output voltage will be too low.  Maintenance method: First, check with a multimeter-whether the low-voltage DC filter capacitor has deteriorated, whether the capacity has decreased, and whether it can be charged and discharged normally. If there is no such problem, measure the current-limiting resistance of the electrode of the switching power tube and the over-current protection resistance of the source to see if the resistance has changed, deteriorated, or opened welding, or has poor contact. If there is no problem, check again—whether the iron core of the high-frequency transformer is intact. In addition to this z, there may be reduced output filter capacitor capacity, or open welding, virtual connection; power output current-limiting resistance variable value or virtual connection; power output line virtual connection, etc.  Don't let go of these difficulties. They should be checked carefully to make sure there is nothing wrong.
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